Question 43985
The equation governing the extension of a spring is {{{F=kx}}}
where k is the stiffness (force per unit length) and F is the force applied. x is the extension of the spring.
A weight of 80 pounds stretches a spring 4.5 inches. So rearranging the equation given, 
{{{k=F/x}}}
{{{k=(80/4.5)pounds per inch}}}
Now when 125 pounds are applied:
{{{125=(80/4.5)x}}}
Rearranging:
{{{x=(125*4.5)/80}}}
the solution is x=7.03 inches (to the nearest 0.01 inch)

I hope this helps.

P.S. I am trying to start my own mathematics homework help website.  I would be extremely grateful if you would send me your feedback on my help to adam.chapman@student.manchester.ac.uk.